Transcript 27_Cooperation

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Social Climbers (Life of Mammals)
Overview
Some of the costs & benefits of cooperation.
Altruism & selfishness.
Relatedness & kin selection.
The Major Transitions
1. Replicating molecules
--->
Molecules in protocells
2. Independent replicators --->
Chromosomes
3. RNA as gene and enzyme
--->
4. Bacteria (prokaryotes)
--->
Eukaryotes (organelles)
5. Asexual clones
--->
Sexual populations
DNA genes, protein enz
6. Single-celled organisms --->
Multicellularity
7. Solitary individuals
--->
8. Primate societies
--->
Eusocial colonies
Human societies (language)
Maynard Smith & Szathmáry 1995
Transition 6
Unicellularity --> Multicellularity
Occurred independently in plants, animals, and
fungi (and others).
Many occurred simultaneously, during the early
Cambrian (~560-600 mya).
Suggests that something happened:
developmental constraint solved.
environmental conditions changed.
Allows division of labor, and specialization on
certain tasks (differentiation).
Problems that needed solving
Differential gene expression
Inheritance of cell identity in differentiated tissues.
Pattern formation.
relative position of cells vs. chemical gradients
Soma vs. germ line
Why be sterile?
The social amoeba, Dictyostelium discoideum
spores
fruiting
body
free-living
amoeba
multicellullar
slug
The social amoeba, Dictyostelium discoideum
spores
fruiting
body
free-living
amoeba
multicellullar
slug
The social amoeba, Dictyostelium discoideum
Cheating can be prevented if:
• Non-relatives are recognized and excluded.
or
• Even mixing occurs within slugs.
Social cheating in Dictyostelium
1. Do unrelated clones mix together to form slugs?
• Mixed two clones together in equal proportions.
• Used microsatellite markers to genotype sections of slugs.
Strassmann et. al (2000) Nature
Social cheating in Dictyostelium
2. Are clones equally represented
in the stalk vs. spore?
• Genotyped and compared prestalk
vs. prespore.
Strassmann et. al (2000) Nature
Social cheating in Dictyostelium
Cheating is not prevented:
• Non-relatives are not excluded.
and
• Even mixing does not occur within slugs.
Some clones appear to be specialized cheaters,
but require wild-type clones for stalk
formation.
The basics of cooperation & conflict
(among cell lineages)
In some systems, cheating can - and does - arise.
Cheaters often do not do well alone (become parasitic)
Bottlenecks increase genetic similarity, and align
interests.
Control can be imposed by previous generation
(maternal effects).
Cell lineages within multicellular organisms
In most organisms, germlines are determined via:
Somatic Embryogenesis
Why be a leaf if you can be an acorn?
Cell lineages within multicellular organisms
In most organisms, germlines are determined via:
Somatic Embryogenesis
Not everyone can become an acorn - plant would die
In most of the organisms we are familiar with,
germlines are preformed or formed early
The unicellular bottleneck
Why break down a costly, multicellular organism
each generation?
Because all cells are genetically identical,
r = 1. This aligns interests.
When cheaters go through bottleneck, they
are non-viable (all acorns).
Other potential benefits:
dispersal
diapause
expose bad alleles
Potential costs:
ecological vulnerability
costs and risks of
development
Transition 7
Solitary individuals to social groups
Dispersal is risky
Death
Finding resources (food, home)
Finding mate
When the costs of ecological constraints are high,
offspring do better by staying home.
Family-based social groups
Risks of not dispersing:
Competing with relatives
Limited resources
Inbreeding
Conflict over reproduction
So an individual’s decision should be determined
by the balance of these costs and benefits.
Potential benefits of sociality
Pooled resources/shared defenses.
Division of labor.
Increase indirect fitness
(by helping relatives reproduce)
Potential costs of sociality
Shared resources
Parasitism
Cheaters
Shared defenses.
dilution effect - prey group together in large numbers
to overwhelm the feeding capacity of the predator.
selfish herd - prey found on the inside of a clump will
tend to survive, those on the outside are eaten.
Prey constantly moving towards middle of a group
and appear flock or school.
kg meat/lion/day
Shared resources
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hunting group size
Caraco and Wolf
Packer
Shared resources
Fieldfare
The larger the colony, the higher probability that some
offspring will die of starvation.
Social spiders (Anelosimus)
Female fitness increases with group size intially,
but then declines in largest groups.
Some re-occurring themes
Relatedness and indirect fitness.
Conflict between selection at different levels.
Why not cheat?
What happens when cheaters succeed?
New levels of individuality (cells, organisms,
colonies) can arise when selection at
lower levels is repressed.